Method, system and apparatus for image-guided insertion of implant devices

ABSTRACT

A method of imaging an implant device in a computing device is provided. The computing device includes a processor interconnected with a memory and a display. The method includes, at the processor: obtaining a first magnetic resonance (MR) image of a patient tissue, the first MR image containing a first magnetic field strength indicator; responsive to the implant device being inserted in the patient tissue, obtaining a second MR image of the patient tissue, the second MR image containing a second magnetic field strength indicator smaller than the first magnetic field strength indicator; registering the second MR image with the first MR image; generating a composite image from the first MR image and the second MR image; and presenting the composite image on the display.

FIELD

The specification relates generally to image-based medical guidancesystems, and specifically to a method, system and apparatus forimage-guided insertion of implant devices.

BACKGROUND

Certain medical devices are employed by implantation into patienttissue. An examples of such devices is a deep brain stimulation (DBS)probe, which is inserted into a patient's brain to deliver electricalpulses to selected anatomical structures. The nature of the insertionprevents the surgeon from directly viewing the position of theelectrodes, and conventional DBS probe insertion procedures thereforerely on medical imaging to confirm that the probe is correctly placed.

The anatomical structures that are relevant to such devices can besmall. For example, DBS probes are often inserted adjacent to thesub-thalamic nucleus (STN) in the brain. Viewing such structures can beaccomplished with various imaging modalities, including MRI. Highermagnetic field strengths (e.g. 3T and above) are particularly effectivefor imaging structures such as the STN. However, the electrodes of DBSprobes and similar components of other implants can interfere with MRimaging, particularly at higher field strengths, producing artifacts anddistortions that render confirmation of the probe's position difficultor impossible.

SUMMARY

According to an aspect of the specification, a method of imaging animplant device is provided in a computing device having a processorinterconnected with a memory and a display. The method comprises, at theprocessor: obtaining a first magnetic resonance (MR) image of a patienttissue, the first MR image containing a first magnetic field strengthindicator; responsive to the implant device being inserted in thepatient tissue, obtaining a second MR image of the patient tissue, thesecond MR image containing a second magnetic field strength indicatorsmaller than the first magnetic field strength indicator; registeringthe second MR image with the first MR image; generating a compositeimage from the first MR image and the second MR image; and presentingthe composite image on the display.

According to a further aspect of the specification, a computing deviceis provided for imaging an implant device, comprising: a memory; adisplay; and a processor interconnected with the memory and the display,the processor configured to perform the above method.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Embodiments are described with reference to the following figures, inwhich:

FIG. 1 depicts a system for image-guided insertion of implant devices,according to a non-limiting embodiment;

FIG. 2 depicts certain internal components of the computing device ofthe system of FIG. 1 , according to a non-limiting embodiment;

FIG. 3 depicts a method of generating guidance images for implant deviceinsertion, according to a non-limiting embodiment;

FIG. 4 depicts an example first image obtained during the performance ofthe method of FIG. 3 , according to a non-limiting embodiment;

FIG. 5 depicts an example second image obtained during the performanceof the method of FIG. 3 , according to a non-limiting embodiment;

FIG. 6 depicts an example composite image generated during theperformance of the method of FIG. 3 , according to a non-limitingembodiment; and

FIGS. 7A and 7B depict MR images of implant devices acquired with lowmagnetic fields, according to a non-limiting embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

As used herein, the terms, “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms,“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

Unless defined otherwise, all technical and scientific terms used hereinare intended to have the same meaning as commonly understood to one ofordinary skill in the art. Unless otherwise indicated, as used herein,the following terms are intended to have the following meanings:

As used herein the term “intraoperative” refers to an action, process,method, event or step that occurs or is carried out during at least aportion of a medical procedure. The term “preoperative” as used hereinrefers to an action, process, method, event or step that occurs or iscarried out before the medical procedure begins. The termsintraoperative and preoperative, as defined herein, are not limited tosurgical procedures, and may refer to other types of medical procedures,such as diagnostic and therapeutic procedures.

FIG. 1 depicts a system 100 for image-guided insertion of implantdevices. System 100, as will be discussed in greater detail below, isconfigured to generate one or more guidance images for useintraoperatively or postoperatively in connection with a procedure toinsert an implant device (not shown) into a tissue of a patient 104. Theimplant device, in general, is a device that, during insertion intopatient 104 may not be directly (i.e. optically) visible to the medicalstaff (e.g. surgeon) performing the insertion.

An examples of such a device is a deep brain stimulation (DBS) probe.DBS probes generally include an array of electrodes (e.g. in a lineararrangement along the length of the probe), and are commonly insertedthrough an opening in the skull of patient 104. Following entry throughthe skull, the probe is inserted through the tissue of the brain untilthe electrodes of the probe are adjacent to a target structure withinthe brain (e.g. the sub-thalamic nucleus (STN)). However, the surgeonmay have limited visibility, or no visibility, of the position of theelectrodes relative to the STN from the opening in the skull of patient104.

Another example of an implant device as discussed herein is a cochlearimplant. As with a DBS probe, the desired insertion location of acochlear implant electrode is adjacent to structures within patient 104(e.g. the cochlea) that are not directly visible to the surgeoninserting the implant.

Various components of system 100 interact to generate theabove-mentioned guidance images. In particular, system 100 includes acomputing device 108 connected to a display 112. Computing device 108 isconfigured to generate the guidance images, and can control display 112to present those images. Computing device 108 generates the guidanceimages based on initial images obtained from at least one imagingdevice. In general, the initial images on which computing device 108operates to generate the guidance images are captured using the sameimaging modality. In the present example, that imaging modality ismagnetic resonance imaging (MRI). In other embodiments, however, thetechniques described herein may be applied to other imaging modalities.

The above-mentioned imaging device is therefore, in the present example,an MRI scanner. More specifically, the initial images on which computingdevice 108 operates to generate the guidance images include imagesacquired at different magnetic field strengths. System 100 thereforeincludes a first MRI scanner 116, and a second MRI scanner 120. MRIscanner 116 has a greater field strength than MRI scanner 120. Forexample, MRI scanner 116 can have a field strength of 7 T, while scanner116 can have a field strength of 0.5 T. A wide variety of other fieldstrengths are also contemplated. For instance, scanner 116 can have afield strength of 3 T while scanner 120 can have a field strength of 1T. As a further example, scanner 116 can have a field strength of 3 Twhile scanner 120 can have a field strength of 0.3 T.

As illustrated in FIG. 1 , scanners 116 and 120 can be installed indifferent areas 124 and 128, respectively, of a building (or in separatebuildings). In some embodiments, scanner 120 can be installed within anoperating theatre for use in capturing intraoperative images. In otherembodiments, however, scanner 120 can be located outside the operatingtheatre, or in the same building or room as scanner 116.

Computing device 108, as will be described in greater detail below, isconfigured to obtain images captured using scanners 116 and 120, and toperform various processing actions to generate guidance images forpresentation on display 112. Before a detailed discussion of theabove-mentioned functionality of computing device 108, a description ofthe components of computing device 108 will be provided.

Referring to FIG. 2 , certain internal components of computing device108 are depicted, including a central processing unit (also referred toas a microprocessor or simply a processor) 202 interconnected with anon-transitory computer readable storage medium such as a memory 204.

Processor 202 and memory 204 are generally comprised of one or moreintegrated circuits (ICs), and can have a variety of structures, as willnow occur to those skilled in the art (for example, more than one CPUcan be provided). Memory 204 can be any suitable combination of volatile(e.g. Random Access Memory (“RAM”)) and non-volatile (e.g. read onlymemory (“ROM”), Electrically Erasable Programmable Read Only Memory(“EEPROM”), flash memory, magnetic computer storage device, or opticaldisc) memory. In the present example, memory 204 includes both avolatile memory and a non-volatile memory. Other types of non-transitorycomputer readable storage medium are also contemplated, such as compactdiscs (CD-ROM, CD-RW) and digital video discs (DVD).

Computing device 108 can also include a network interface 206interconnected with processor 202. Network interface 206 allowscomputing device 108 to communicate with other devices via a network(e.g. a local area network (LAN), a wide area network (WAN) or anysuitable combination thereof). Network interface 206 thus includes anynecessary hardware for communicating over such networks, such as radios,network interface controllers (NICs) and the like. The devices withwhich computing device 108 can communicate are not particularly limitedin nature, and can include other computing devices, scanners 116 and120, and the like.

Computing device 108 can also include an input/output interface 208,including the necessary hardware for interconnecting processor 202 withvarious input and output devices. Interface 208 can include, among othercomponents, a Universal Serial Bus (USB) port, an audio port for sendingand receiving audio data, a Video Graphics Array (VGA), Digital VisualInterface (DVI) or other port for sending and receiving display data,and any other suitable components.

Via interface 208, computing device 108 is connected to input devicesincluding a keyboard and mouse 210, a microphone 212, as well as outputdevices, including display 112. Computing device 108 can also beconnected to devices such as a tracking system (e.g. an infrared-basedoptical tracking system), imaging scopes, lighting systems and the like.Those components, however, are not directly relevant to the presentdiscussion and are therefore not illustrated. Other input (e.g. touchscreens) and output devices (e.g. speakers) will also occur to thoseskilled in the art.

It is contemplated that I/O interface 208 may be omitted entirely insome embodiments, or may be used to connect to only a subset of thedevices mentioned above. The remaining devices may be connected tocomputing device 108 via network interface 206.

Computing device 108 stores, in memory 204, an image processingapplication 216 (also referred to herein as application 216) comprisinga plurality of computer readable instructions executable by processor202. When processor 202 executes the instructions of application 216(or, indeed, any other application stored in memory 204), processor 202performs various functions implemented by those instructions, as will bediscussed below. Processor 202, or computing device 108 more generally,is therefore said to be “configured” or “operating” to perform thosefunctions via the execution of application 216.

Also stored in memory 204 are various data repositories, including apatient data repository 218. Patient data repository 218 can contain asurgical plan defining the various steps of a surgical procedure to beconducted on patient 104 (such as the implantation of a DBS probe), aswell as image data relating to patient 104, such as initial imagescaptured using scanners 116 and 120, as well as guidance imagesgenerated by computing device 108 based on those initial images.

As mentioned above, computing device 108 is configured, via theexecution of application 216 by processor 202, to generate guidanceimages based on images obtained from scanners 116 and 120, for use inthe insertion of implant devices (such as DBS probes or cochlearimplants) into tissues of patient 104. The actions performed bycomputing device 108 to generate such images will be described in detailbelow.

Turning to FIG. 3 , a method 300 of generating guidance images forimplant device insertion is illustrated. Method 300 will be described inconjunction with its performance in system 100. The blocks of method300, as described below, are performed by computing device 108, and morespecifically, by processor 202 via the execution of application 216. Inother embodiments, method 300 can be performed within other suitablesystems.

At block 305, computing device 108 is configured to obtain a firstimage—in the present embodiment, a first MR image of a tissue of patient104—containing a first magnetic field strength indicator. As will beapparent to those skilled in the art, images acquired with scanners 116and 120 can include metadata, such as Digital Imaging and Communicationsin Medicine (DICOM) data specifying various imaging parameters. Thefield strength indicator contained in the first image indicates thestrength of the magnetic field emitted by the scanner used to acquirethat image.

The first image obtained at block 305 can be obtained via retrieval frommemory 204 (e.g. from repository 218), having been captured at anearlier time via the control of scanner 116 by another computing device.In other embodiments, the first image can be obtained at block 305directly from scanner 116. In other words, computing device 108 can beconfigured to control scanner 116 to capture the first image in someembodiments.

The first image is acquired prior to the insertion of the implantdevice, and is generally acquired preoperatively. The first imagetherefore depicts patient tissue, but does not depict any portion of theimplant device. As noted above, the first image can be acquired usingscanner 116, following which patient 104 may be transported to anoperating theatre such as that contained in area 128 illustrated in FIG.1 .

At block 310, responsive to insertion (or at least partial insertion) ofthe implant device into the tissue of patient 104, computing device 108is configured to obtain a second image—in the present embodiment, asecond MR image—of the patient tissue. As with the first image, thesecond image can be obtained either directly from an imaging deviceunder the control of computing device 108, or from memory 204 (havingbeen stored in memory 204 previously) or from another computing devicevia network interface 206. The second MR image contains a secondmagnetic field strength indicator smaller than the first magnetic fieldstrength indicator. In other words, the second image is acquired with ascanner having a lower field strength than the scanner with which thefirst image was acquired. In the present example, the first image can beacquired with scanner 116 (e.g. a 7 T scanner), while the second imagecan be acquired with scanner 120 (e.g. a 0.5 T scanner).

The second image is acquired either intraoperatively (i.e. during theprocedure to insert the implant device into patient 104) orpostoperatively (i.e. after the procedure is complete). More generally,the second image is acquired after the insertion of the implant devicehas begun, and thus the second image, as obtained by computing device108, depicts at least a portion of the implant device.

FIG. 4 depicts an example first image 400 of the brain of patient 104,as obtained at block 305. FIG. 5 , meanwhile, depicts an example secondimage 500 of the brain of patient 104, as obtained at block 310. As seenby comparing images 400 and 500, image 400 depicts the patient tissue ata greater level of detail than image 500. This can arise because, forexample, the higher field strength used to acquire first image 400 canprovide a greater signal-to-noise ratio. The increased level of detailvisible in first image 400 may also result from increased iron contentin certain fine anatomical structures. For example, the STN can have anelevated iron content in comparison with surrounding issues, and thus isless suitable for imaging at lower magnetic field strengths (i.e. isless easily viewed in images such as image 500).

It will also be noted that image 500, although depicting patient tissuein less detail than image 400, depicts a portion of an implant device504. In the present example, device 504 is a DBS probe including aplurality of electrodes 508. As noted earlier, imaging a device such asDBS probe 504 under a greater magnetic field such as that used toacquire image 400 can result in distortions and artifacts, up to andincluding complete signal loss in the region surrounding probe 504.

Images 400 and 500 can be acquired using any suitable MR protocol (e.g.gradient echo based sequences). In a preferred embodiment, images 400and 500 are quantitative images, such as T1 maps or T2 maps, whoseacquisition will be familiar to those skilled in the art. In a T1 mapimage, each pixel or voxel has a value that indicates the absolutespin-lattice relaxation time (T1) for the tissue depicted by that pixelor voxel. In a T2 map image, each pixel or voxel has a value thatindicates the absolute spin-spin relaxation time (T2) for the tissuedepicted by that pixel or voxel.

Returning to FIG. 3 , having obtained the two images at block 305 and310, computing device 108 is configured to register the images at block315. Image registration refers to the process of placing both images ina common coordinate system, such that any given set of coordinates inthe common system identifies portions of both images depicting the samearea of patient 104. In general, each obtained image begins with animage-specific coordinate system. For example, a two-dimensionalpreoperative image may have a coordinate system in which the origin liesat the lower-left corner of the image. A two-dimensional intraoperativeimage may also have a coordinate system in which the origin lies at thelower-left corner of the image. However, because the two images may notdepict exactly the same area of patient 104, the two coordinate systemscannot be directly compared. That is, the pixel located at (1200, 205)in image 400 may depict an entirely different portion of patient 104than the pixel at (1200, 205) in image 500.

Therefore, in order to align images 400 and 500 on a common coordinatesystem, computing device 108 is configured to generate and apply atransformation operation is to at least one of the images. In thepresent example, it is contemplated that the transformation operation isto be applied to image 500, but the transformation operation can also beapplied to image 400 in other embodiments. The nature of thetransformation is not particularly limited, and a variety of imageregistration algorithms will occur to those skilled in the art fordetermining such transformation operations. In general, thetransformation operation manipulates the pixels or voxels of one or bothof the images (e.g. by translation, rotation, distortion, scaling, andthe like) to place the pixels or voxels in the common coordinate system.

The transformation can be determined by computing device 108 bycomparing images 400 and 500 and identifying common features between theimages, such as edges, intensities, and other image features (includinganatomical features such as sulci and ventricles). Transformationparameters (e.g. scaling, rotation, localized deformations and the like)can be optimized by computing device 108 according to conventionalalgorithms to place such features in alignment.

In embodiments in which images 400 and 500 are T1 maps or T2 maps, imageregistration at block 315 may be accelerated by permitting the use ofmutual information based image registration processes, because bothimages contain absolute measurements that can be compared directly(rather than image attributes such as contrast or colour, which are notnecessarily directly comparable). Examples of image registrationalgorithms applicable to quantitative images such as T1 and T2 mapsinclude quasi-Newton algorithms, simultaneous perturbation algorithms,and the like.

Upon completion of block 315, any given coordinate set of image 400depicts the same (or substantially the same) region of patient tissue asthe same coordinate set in image 500. At block 320, computing device 108is configured to generate a composite image from the registered firstand second images 400 and 500. In general, the composite image depictsthe implant device as shown in second image 500, and patient tissue asshown in first image 400. Therefore, computing device 108 is configuredto generate the composite image by selecting a region of second image500 depicting the implant device, and overlaying the selected region ofsecond image 500 on first image 400.

The selection of a region of second image 500 for use in the compositeimage at block 320 can be performed in various ways. For example,computing device 108 can be configured to select, for use in thecomposite image, any pixel or voxel from second image 500 having a value(e.g. a T1 value) that is different from the value in image 400 for thesame location by at least a predefined threshold. If the difference inpixel or voxel values between images 400 and 500 for a given pixel orvoxel does not exceed the threshold, the pixel or voxel from image 400is placed in the composite image instead of the pixel or voxel fromimage 500. In other words, regions of the two images having low levelsof mutual information can be identified during image registration, andin the composite image, data from image 400 in those regions can beplaced instead of data from image 500. In some embodiments, theabove-mentioned threshold can be altered, for example via input datareceived from keyboard/mouse 210.

FIG. 6 illustrates a composite image 600 resulting from the performanceof block 320. As seen in FIG. 6 , probe 504 and electrodes 508 arevisible in composite image 600; however, the remainder of image 600contains data from image 400, and thus certain fine anatomicalstructures are visible to a greater degree in composite image 600 thanin second image 500.

Referring again to FIG. 3 , at block 325 computing device 108 isconfigured to present composite image 600 on display 112 (or any othersuitable display connected to computing device 108). Following theperformance of block 325, computing device 108 can be configured todetermine whether to obtain a further intraoperative or postoperativeimage at block 330. For example, the determination at block 330 caninclude a determination as to whether input data has been receivedrepresenting a command to obtain a further intraoperative orpostoperative image. Such further images can be acquired following anintraoperative adjustment to the position of implant 504, orpostoperatively to assess whether implant 504 has shifted within patient104.

When the determination at block 330 is negative, the performance ofmethod 300 ends. When, on the other hand, the determination at block 330is affirmative, computing device 108 is configured to repeat theperformance of blocks 310 to 325 to acquire a third image (e.g. a thirdMR image having a lower field strength indicator than first image 400),register the third image with first image 400, and generate a furthercomposite image based on first image 400 and the third image. The thirdimage can have the same field strength indicator as second image 500(that is, the third image has a field strength indicator equal to thefield strength indicator of second image 500), although this is notmandatory—the third image can be acquired using a different scanner thansecond image 500, and thus can be acquired at a different fieldstrength, so long as the field strength of the third image remains lowerthan that of first image 400.

Turning briefly to FIGS. 7A and 7B, examples of second images (obtainedat block 310) are shown to illustrate the largely distortion-freecharacteristics of images captured at reduced magnetic field strengths.FIG. 7A is an MRI scan of a cochlear implant embedded in a uniformgelatin-based medium, and FIG. 7B is an MRI scan of a DBS probe embeddedin a similar gelatin-based medium. The images of both FIGS. 7A and 7Bwere acquired at field strengths of about 0.5 T.

Variations to the above are contemplated. For example, in someembodiments the images obtained at blocks 305 and 310 can be acquiredusing techniques such as diffusion tensor imaging (DTI), and can thusrepresent diffusion tracts within patient tissue (e.g. the brain ofpatient 104). The diffusion tracts (e.g. their orientation, position anddimensions) can be employed at block 315 as features for imageregistration.

The scope of the claims should not be limited by the embodiments setforth in the above examples, but should be given the broadestinterpretation consistent with the description as a whole.

We claim:
 1. A method of imaging an implant device in a computing devicehaving a processor interconnected with a memory and a display,comprising, at the processor: obtaining a first pre-operative magneticresonance (MR) image of a patient tissue, the first pre-operative MRimage containing a first magnetic field strength indicator; afterinsertion of the implant device in the patient tissue, obtaining asecond intra-operative MR image of the patient tissue and the implantdevice, the second intra-operative MR image containing a second magneticfield strength indicator smaller than the first magnetic field strengthindicator; registering the second intra-operative MR image with thefirst pre-operative MR image; generating a composite image from thefirst pre-operative MR image and the second intra-operative MR image,wherein generating the composite image includes: for each voxel of thesecond intra-operative MR image, (i) determining a difference between avalue of the voxel and a value of a corresponding voxel of the firstpre-operative MR image, (ii) determining whether the difference excedesa threshold and (iii) when the difference exceeds the thresholdselecting the voxel of the second intra-operative MR image as indicatinga presence of the implant device; and substituting the selected voxelsof the second intra-operative MR image for the corresponding voxels ofthe first pre-operative MR image; and presenting thee composite image onthe display.
 2. The method of claim 1, wherein the first pre-operativeMR image and the second intra-operative MR image each comprises aquantitative T1 map image.
 3. The method of claim 1, wherein the firstpre-operative MR image and the second intra-operative MR image eachcomprises a quantitative T2 map image.
 4. The method of claim 1, whereinthe first pre-operative MR image and the second intra-operative MR imageeach comprises a diffusion tensor imaging (DTI) image.
 5. The method ofclaim 1, wherein the implant device comprises one of (i) a deep brainstimulation (DBS) probe and (ii) a cochlear implant.
 6. The method ofclaim 1, further comprising: obtaining a third MR image of the patienttissue, the third MR image containing a third magnetic field strengthindicator smaller than the first magnetic field strength indicator. 7.The method of claim 6, the third magnetic field strength indicator beingequal to the second magnetic field strength indicator.
 8. The method ofclaim 6, further comprising: registering the third MR image with thefirst pre-operative MR image; generating a further composite image fromthe first pre-operative MR image and the third MR image; and presentingthe further composite image on the display in place of the compositeimage.
 9. The method of claim 6, further comprising: obtaining the thirdMR image after a positional adjustment of the implant device within thepatient tissue.
 10. A computing device for imaging an implant device,comprising: a memory; a display; and a processor interconnected with thememory and the display, the processor configured to: obtain a firstpre-operative magnetic resonance (MR) image of a patient tissue, thefirst pre-operative MR image containing a first magnetic field strengthindicator; after insertion of the implant device in the patient tissue,obtain a second intra-operative MR image of the patient tissue and theimplant device, the second intra-operative MR image containing a secondmagnetic field strength indicator smaller than the first magnetic fieldstrength indicator; register the second intra-operative MR image withthe first pre-operative MR image; generate a composite image from thefirst pre-operative MR image and the second intra-operative MR image,wherein generation of the composite image includes: for each voxel ofthe second intra-operative MR image: (i) determining a differencebetween a value of the voxel and a value of a corresponding voxel of thefirst pre-operative MR image, (ii) determining whether the differenceexceeds a threshold, and (iii) when the difference exceeds thethreshold, selecting the voxel of the second intra-operative MR image asindicating a presence of the implant device; and substituting theselected voxels of the second intra-operative MR image for thecorresponding voxels of the first pre-operative MR image; and presentthe composite image on the display.
 11. The computing device of claim10, wherein the first pre-operative MR image and the secondintra-operative MR image each comprises a quantitative T1 map image. 12.The computing device of claim 10, wherein the first pre-operative MRimage and the second intra-operative MR image each comprises aquantitative T2 map image.
 13. The computing device of claim 10, whereinthe first pre-operative MR image and the second intra-operative MR imageeach comprises a diffusion tensor imaging (DTI) image.
 14. The computingdevice of claim 10, wherein the implant device comprises one of (i) adeep brain stimulation (DBS) probe and (ii) a cochlear implant.
 15. Thecomputing device of claim 10, the processor further configured to:obtain a third MR image of the patient tissue, the third MR imagecontaining a third magnetic field strength indicator smaller than thefirst magnetic field strength indicator.
 16. The computing device ofclaim 15, the third magnetic field strength indicator being equal to thesecond magnetic field strength indicator.
 17. The computing device ofclaim 15, the processor further configured to: register the third MRimage with the first pre-operative MR image; generate a furthercomposite image from the first pre-operative MR image and the third MRimage; and present the further composite image on the display in placeof the composite image.
 18. The computing device of claim 15, theprocessor further configured to: obtain the third MR image after apositional adjustment of the implant device within the patient tissue.